The invention relates to tubular cores onto which pressure-sensitive tape material is wound. The invention relates more particularly to such cores made of fibrous material such as paper board and to methods of making such cores, in which the cores are provided with an outermost layer of material that allows the tape material directly in contact with the core to be released cleanly from the core without being contaminated by fibers from the paper board body of the core.
Pressure-sensitive tapes are commonly wound onto cores formed from paper board. When the tail end of the tape that is directly in contact with the paper surface of the core is peeled off the core, some of the paper fibers stick to the adhesive side of the tape, which renders the tail end unsuitable for use.
A number of techniques have been tried or proposed for eliminating this problem of contamination of the tail end of the tape. One approach has been to form the core with an outermost paper board ply whose outer surface is coated with a release material that allows the tape to be peeled off. With this method, it is difficult to assure that no uncoated paper will be exposed at the outer surface of the core. For example, if the release-coated ply is wound such that there is a gap between the juxtaposed opposite edges of the ply, then uncoated paper board of the underlying ply is exposed in this gap. The gap can potentially be eliminated by forming perfect butt joints between the juxtaposed edges of the ply, but in practice this is very difficult to do, particularly at high winding speeds. Thus, this is not a feasible solution to the problem.
One solution to the problem of the gap between the ply edges that has been attempted is to first wind a relatively narrow strip of material having a release surface onto the core at the same helical pitch that the outermost release-coated ply is to be wound, and then the outermost ply is wound onto the core such that the juxtaposed edges of the ply fall on the underlying narrow strip. This is a relatively complicated and expensive method.
Another way of eliminating the gap is to overlap the opposite edges of the release-coated outermost ply and bond the overlapping edges together with adhesive. The problem with this technique is that the release coating on the outer surface of the ply makes it difficult to achieve a firm bond between the overlapping edges of the ply. Additionally, the uncoated side edge of the overlapping ply is still exposed.
Another method that has been tried involves winding a non-paper film (e.g., cellophane, polypropylene, or the like) onto the paper core and adhering it to the core such that the outer surface of the core is completely covered by the film. The film edges can be overlapped to ensure that no paper fibers are exposed at the outer surface of the core. However, when the core is cut to desired lengths, the film overlap joint tends to delaminate, which can expose paper fibers at the cut end of the core.
The above needs are met and other advantages are achieved by the present invention, which provides cores and methods for making cores in which a release-coated outer ply is wound onto and adhered to the core so as to completely cover the fibrous plies of the core. The release-coated ply is wound with a special overlap joint that ensures a good bond between the overlapping edges of the ply. In one embodiment of the invention, the release-coated ply comprises a strip of polymer film having a release coating on its outer surface and a tacky adhesive layer on its inner surface. The strip is wound onto the core with one edge of the strip folded outwardly away from the core so as to expose the tacky adhesive layer on the folded edge. This folded edge is overlapped by the opposite edge of the strip, thereby placing the tacky adhesive layers on the two edges in contact with each other. A firm bond between the overlapping edges is achieved by the adhesive-to-adhesive contact. In a preferred embodiment, the overlapping edge of the strip overlaps slightly beyond the folded edge onto the unfolded outer surface of the strip. This unfolded portion is treated to promote adhesion of the overlapping edge thereto. For example, the release coating on this portion can be corona discharge treated to destroy the release coating. The polymer strip advantageously can be formed of polypropylene with a pressure-sensitive adhesive applied on one side and a silicone-based release coating or the like on the other side. This material has the advantages of being readily available and inexpensive.
Improved bonding of the polymer strip to the core, and improved conformance of the strip to irregularities in the core surface, are achieved in a preferred embodiment by heating the strip as it is being advanced to the core and applying tension to the heated strip to cause it to stretch as it is wound onto the core. It has been found that the strip, when so heated and stretched, conforms well to slightly rough core surfaces. This effect can be advantageous in that the resulting surface of the final core is not perfectly smooth, and hence there may be less contact area between the tape and the core so that the tape releases more easily from the core.
In an alternative embodiment of the invention, the outermost ply of the core is formed of a strip of fibrous material such as paper board, one side of which is coated with a polymer film. A release coating is applied over the side of the polymer film facing away from the paper board. The strip of coated paper board is wound onto the core with adhesive interposed therebetween. One edge of the strip is folded inwardly toward the core, and this folded edge overlaps the opposite unfolded edge of the strip, thereby placing the film on the folded edge in contact with the film on the opposite edge. The overlapping edges are bonded together. Preferably, the film on the strip is a heat-sealable material, and the edges are bonded together by heat sealing. Advantageously, the adhesive for bonding the strip to the core is applied to the inner surface of the strip before it is wound and before the edge of the strip is folded such that the adhesive bonds the folded edge onto the inner surface of the strip to prevent unfolding.